This invention relates to pressure transducers in general and more particularly to an environmentally protected pressure transducer configuration which employs interconnected or shunted resistive bridge arrays for providing a pressure output indicative of a pressure relationship between two different pressure ports.
Pressure transducers have been utilized in many applications whereby the external environment imposes difficult operating conditions for the transducer structure. Such transducers, for example, have been employed in aircraft, automobiles and other vehicles. In such units the transducer is constantly exposed to moisture, fuel, solvents, hydraulic fluids and the elements in general. These transducers are associated with pressure ports as well as with internal cavities. During such operation, the cavities as well as the ports may accumulate excessive amounts of water. This water can harm the silicon or metallic elements employed in conventional transducers. Metal diaphragms are often employed either as isolation diaphragms or with strain gages mounted on the interior surface of the diaphragm to provide the required media isolation. Such an approach is entirely satisfactory for absolute or sealed gage transducers, however, for gage or differential transducers, severe problems arise.
For a device with strain gages on a metal diaphragm, the gages are exposed to the reference environment in such transducers. Various techniques, including coating with organics, have been tried but none have been found to offer full protection. Metal isolation using sensor capsules in a sealed cavity device pose another problem. Such devices employ a contained volume of oil which must be hermetically sealed within the pressure cavity. Such a seal is difficult to obtain in a gage sensor employing a silicon diaphragm. It is very difficult to form a hermetic and stress free seal for a silicon diaphragm sensor. Thus water and other deleterious substances may render the device inaccurate or actually destroy the piezoresistive elements or interconnecting metallizations which are intimately associated with the transducers used in these applications.
In transducers used for such applications the devices employed may constitute gage or differential pressure transducers requiring two separate ports such as a pressure input port and a negative pressure or reference port. A single diaphragm may be employed with the positive pressure applied on one side of the diaphragm and the gage or reference pressure applied on the second side of the diaphragm. Thus, the device responds to the difference between the pressure applied to the positive side and the pressure applied to the negative side of the diaphragm. This invention employs two absolute pressure sensors each with a sealed vacuum reference. One transducer is exposed to the pressure to be measured and one transducer is exposed to a reference pressure. The outputs from the transducer elements of such devices must be tailored to provide a linear output for a unit of applied pressure. In order to add the outputs and to provide compensation, additional circuitry such as operational amplifiers, resistive compensation techniques and so on must be employed. Such additional components as were intimately associated with the transducer stucture were also subject to destruction and contamination by the external environment. External or internal active components in general do a less than ideal job of integrating two transducers into a single differential or gage unit.
As can be ascertained, many different industries such as the aircraft industry as well as others require gage pressure transducers. These devices have an output which is referenced to atmospheric pressure or where the output is referenced to a second reference pressure which again may be atmospheric pressure or another time varying reference pressure. As such, differential pressure measurements can be accomplished by many different transducer structures.
One such structure is a differential transducer which provides an output which is the difference between two pressures. In the case of a gage sensor, one of these pressures, referred to here as the reference pressure, is atmospheric pressure and the other pressure is the pressure being monitored or the measured pressure. Essentially, a gage transducer produces an output which is the difference between atmospheric pressure and a monitored pressure. In the case of absolute pressure transducers, the output is directly indicative of the monitored pressure as referenced to vacuum. Essentially, as is well known, many of such transducers employ piezoresistive sensors. These sensors are usually arranged in a bridge pattern of resistors which are mounted or diffused on a thin diaphragm member. The diaphragm member which may be fabricated from silicon flexes upon application of pressure thereto causing fiber stresses on the top surface which elongate or shorten the piezoresistors causing them to vary their resistance according to the deflection of the diaphragm. In regard to such devices, one may use a single diaphragm where one side of the diaphragm is exposed to a first pressure and the other side of the diaphragm is exposed to a second pressure. It is, of course, understood that in such devices the diaphragm will have the piezoresistive pattern arranged on one side, and, therefore, the pattern will be exposed to either the measured pressure or the reference pressure or atmospheric pressure which may serve to alter undesirably the characteristics of the piezoresistors or otherwise affect their operation.
Silicon diaphragm sensors in general have a robust side and non-robust side. The side with the piezoresistors is sensitive to many environments such as moisture and must be suitably protected. Many schemes have been used for protection but most are imperfect and introduce various problems. In general the best protection is provided by a metal isolation diaphragm coupling the pressure through a fluid transmitting medium. Such an approach introduces two problems, both of which are solved by this invention. The first problem is that it is difficult to make a gage or differential transducer using such an approach. This is because the oil must be sealed in an absolutely hermetic manner. Such a seal is difficult to accomplish with a silicon chip. The second problem is that the trapped oil expands and causes an unwanted pressure signal. This problem is solved by novel interconnection circuits.
In order to avoid this, the prior art employed two separate transducers, each of which was associated with its own pressure port or aperture. Each of these devices contained a separate bridge which, therefore, produced a voltage output proportional to the pressure at each of the respective ports. In order to obtain a proper voltage output, the separate bridge outputs had to be combined by means of electronic circuitry such as suitable operational amplifiers, impedance networks, and so on to derive a voltage which indicated a pressure at one port in relation to the pressure at the other port. These circuits are relatively complicated and include many different components to derive the final output voltage. Prior art devices employing dual diaphragms have been found to be uneconomical and not suitable for aircraft applications.
An additional consideration, in regard to these prior art techniques is that the use of additional circuit components to derive the desired voltage further resulted in complicated compensation techniques which created additional problems. Certain other approaches to the problem utilized a half bridge configuration from each of two separate transducers to derive an absolute pressure output from two separate diaphragm structures. An example of such a device may be had by referring to U.S. Pat. No. 4,222,277 entitled MEDIA COMPATIBLE PRESSURE TRANSDUCER, issued on Sept. 16, 1980 to Anthony D. Kurtz and Joseph R. Mallon, Jr. and assigned to the assignee herein.
In this patent there is shown an absolute pressure transducer which contains two separate sensor configurations on a common diaphragm. The device is suitable for measuring a benign pressure with reference to a relatively corrosive pressure. The top surface of the wafer is exposed to a relatively clean source of pressure while the gage sensor portion of the diaphragm responds to two sources of pressure. The gage configurations associated with each of the diaphragm sections are then coupled together to form a full bridge array. The full bridge is implemented by a half bridge array used from each separate sensor configuration. Essentially, the structure shown provides a reduced output as a half bridge provides half the output of a full bridge configuration. This device is not suitable for applications where both the ports must be exposed to a difficult medium. It requires at least one relatively clean port. Thus in many of the prior art approaches, a half bridge configuration was employed on each of two separate sensors and then the half bridge structures were wired to produce a full bridge output.
It is desirable to utilize a full bridge array to respond to relatively high pressures and to obtain a maximum output in regard to such pressures. One of the embodiments to be described provides a full bridge sensor at the positive port allowing twice the output obtainable from a half bridge transducer.
An additional consideration, apart from the above noted problems is the further fact that such transducers which essentially reference two different pressure sources must generally contain a pressure transmitting fluid such as oil which fluid must be isolated from the pressure transmitting environment and which environment must be isolated from the semiconductor transducer.
It is an object of the present invention to provide an environmentally compensated pressure transducer structure which structure prevents contamination of the transducer sensor elements. This invention seeks to provide a mechanical structure which is practical and economical for implementation into a transducer suitable for aircraft or general purpose measurement. Dual diaphragm transducers of previous art were impractical because of their expensive and bulky design and because of the difficulty of interconnecting two transducers to form a single transducer of high accuracy.
It is, therefore, an object of the present invention to utilize a full bridge arrangement which full bridge arrangement is capable of monitoring variations in a pressure source of a relatively large magnitude. In order to provide a transducer which will correct for an ambient pressure, the full bridge arrangement has at least two of its resistors responsive to pressure input to the positive port and two of its resistors responsive to pressure applied to the negative port which may monitor an ambient pressure such as atmospheric pressure. In this manner, the magnitude of the resistors in the half bridge array are selected such that the combined output from the bridge structure will produde a voltage which is indicative of the pressure applied to the full bridge array as modified by the ambient pressure as applied to the half bridge array.